The best explanation of private browsing comes from this paper "Unwanted information from the user’s browsing history will not appear in the history bar, auto-filled forms, or search suggestions. Private modes thus help users hide browsing sessions from people sharing a device with them"
Private Browsing does not 100% prevent:
- Geolocation tracking
- Malware protection
- Advertising elimination
- Tracking by websites you visit
Many people have a false understanding that private browsing will magically protect you from advertisers and spyware. Firefox does allow blocking while in private mode, but these are two different features.
Google chrome will still track you when using incognito mode.